The invention relates to improving the performance of a comparator circuit. Typically a comparator has a reference input and a signal input. When the signal input exceeds the reference, the circuit generates an output signal. All comparators require that the signal exceed the reference by an increment that represents an error value. Typically this error is reduced to an acceptable value by employing sufficient gain in the comparator amplifying portions. In many applications it is difficult or inconvenient to simply increase gain. For example, if a low supply potential is involved, gain stages cannot be cascaded as desired. Low supply potentials also often mean that low current drain is a desired feature and current starvation is employed. This too acts to reduce transistor gain.
One important use of comparator circuits is in current-to-time transducers. Typically a capacitor is connected across the signal input to a comparator. The current to be sensed is coupled to charge the capacitor. If the applied current is constant, the capacitor charge is a precisely linear rising voltage. When the voltage exceeds the comparator reference voltage, a trip point is reached where an output is produced. Since the voltage rise is linear, the time to trip is a very precise analog of the current. As a practical matter the current need not be constant if the requirement is that the time to trip is the integral of the current. In actual practice the integral of the current can be made to produce a very precisely related time interval over many orders of magnitude.
One large scale use of such circuitry is in the control of automatic cameras. It has been found that silicon photodiodes have a current response to light that is linear from full sunlight to dimly illuminated night scenes. This range can involve 5 to 6 orders of magnitude. Typical photocurrents for reasonable sized elements can range form 10 microamperes to 10 picoamperes. In terms of the time conversion, a 50 pf capacitor will be charged to one volt level in from about 5 seconds at 10 pa to 5 microseconds at 10 microamperes. In the camera control application the photodiode is exposed to the illumination of the scene to be photographed as the shutter is opened. The photocurrent is integrated until the point is reached where the film is properly exposed, whereupon the trip point is reached and the signal from the comparator used to close the shutter. Such control systems have proven to be very effective and car be manufactured using conventional bipolar integrated circuit techniques.
For example, the light to current conversion is shown in the copending application of Dennis M. Monticelli, Ser. No. 707,745, filed July 22, 1976, and titled PHOTODIODE OPERATIONAL AMPLIFIER. The device described in that application will provide a current output linearly related to light level. A photodiode is disclosed that operates in the 7.5 microampere to 100 picoampere range.
When bipolar transistors are used in comparators, leakage currents and base current inputs in the picoampere range are common. Therefore in the prior art many such circuits would not operate reliably at the lower current values. In the worst case, if the current to be integrated is smaller than the input current required to trip the comparator, the comparator will never trip. In the camera control operation this means that the shutter will remain open and the dimly lit scene grossly overexposed.